Liquid crystal displays (LCDs) are flat panel display devices that include flat glass substrates or sheets. The fusion process is a preferred technique used to produce sheets of glass used in LCDs because the fusion process produces sheets whose surfaces have superior flatness and smoothness compared to sheet produced by other methods. The fusion process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
Many of the glasses manufactured for flat panel display applications, particularly those formed by fusion processes (also referred to as the downdraw or slot draw processes), are melted or formed using components made from refractory metals, e.g. platinum or platinum alloys. This is particularly true in the fining and conditioning sections of the process, where refractory metals are employed to minimize the creation of compositional inhomogeneities and gaseous inclusions caused by contact of the glass with oxide refractory materials. In addition, many of these manufacturing processes employ arsenic as a fining agent. Arsenic is among the highest temperature fining agents known, and, when added to the molten glass bath, it allows for O2 release from the glass melt at high melting temperatures (e.g., above 1450° C.). This high temperature O2 release, which aids in the removal of bubbles during the melting and fining stages of glass production, coupled with a strong tendency for O2 absorption at lower conditioning temperatures (which aids in the collapse of any residual gaseous inclusions in the glass), results in a glass product essentially free of gaseous inclusions.
From an environmental point of view, it would be desirable to provide alternative methods of making such high melting point and strain point glasses without having to employ arsenic as a fining agent. It would be particularly desirable to find methods for making such glasses via fusion processes. Unfortunately, previous efforts at doing so have been hindered by the production of unacceptable amounts of bubbles in the glass. This has been a particular problem in molten glass systems that employ refractory metals such as platinum or platinum-containing alloys because metals such as platinum (and molybdenum) can cause an electrochemical reaction to occur with the glass which results in bubble formation at the glass/platinum interface, i.e. where the glass contacts the platinum. This bubble formation in the glass/platinum contact regions is also referred as surface blistering.
U.S. Pat. No. 5,758,726 discloses a process for reducing bubbles in glass sheets formed in systems utilizing platinum-containing vessel. The process involves controlling the partial pressure of hydrogen outside the vessel relative to the partial pressure inside the vessel. Another approach for reducing bubbles in glass sheets is disclosed in U.S. Pat. Nos. 6,128,924 and 5,824,127, which disclose the use of various batch constituents to minimize the water content in the glass composition and thus the hydrogen concentration on the inside surface of the platinum-containing melter wall. Although the methods disclosed in the patents mentioned above successfully reduce bubbles in glass sheets formed in systems utilizing platinum-containing components, it would be desirable to provide alternative methods to prevent surface blistering on glass sheets.